there are a couple of spots where the numbers got skipped on purpose, without a project being proposed for the gaps, the P-73 and P-74 spots were skipped so that the Eagle could be P-75, Curtiss never proposed any F5C, that was skipped on purpose, etc.
as for kit availability, that depends on the scale,,,,,I leave it up to each modeler to propose a model that is not on this census of kits,,,,,,the "never kitted" types are rare,,,,,,this is 1/72 only (I don't know if anyone ever did a 1/48 site) www.72scale.com/menu.htm
But, even if you stick to one scale like I do, if you expand your scope when you finish a bunch of kits, you can model forever and never reach the "end" of our scale. A lack of kit production only comes up when you need to zero in on one certain version of an aircraft and no one has done one yet. Then you need to do what Don did with the Link Trainer,,,,,build your own without a kit.
Rex